.TH tcpsynbl 8  "2019-07-03" "USER COMMANDS"
.SH NAME
tcpsynbl \- Show the TCP SYN backlog as a histogram. Uses bpftrace/eBPF.
.SH SYNOPSIS
.B tcpsynbl
.SH DESCRIPTION
This tool shows the TCP SYN backlog size during SYN arrival as a histogram.
This lets you see how close your applications are to hitting the backlog limit
and dropping SYNs (causing performance issues with SYN retransmits), and is a
measure of workload saturation. The histogram shown is measured at the time of
SYN received, and a separate histogram is shown for each backlog limit.

This works by tracing the tcp_v4_syn_recv_sock() and tcp_v6_syn_recv_sock()
kernel functions using dynamic instrumentation. Since these functions may
change in future kernels, this tool may need maintenance to keep working.

Since this uses BPF, only the root user can use this tool.
.SH REQUIREMENTS
CONFIG_BPF and bpftrace.
.SH EXAMPLES
.TP
Show the TCP SYN backlog as a histogram.
#
.B tcpsynbl.bt
.SH FIELDS
.TP
backlog
The backlog size when a SYN was received.
.TP
count
The number of times this backlog size was encountered.
.TP
distribution
An ASCII visualization of the count column.
.SH OVERHEAD
Inbound SYNs should be relatively low compared to packets and other events,
so the overhead of this tool is expected to be negligible.
.SH SOURCE
This tool originated from the book "BPF Performance Tools", published by
Addison Wesley (2019):
.IP
http://www.brendangregg.com/bpf-performance-tools-book.html
.PP
See the book for more documentation on this tool.
.PP
This version is in the bpftrace repository:
.IP
https://github.com/iovisor/bpftrace
.PP
Also look in the bpftrace distribution for a companion _examples.txt file
containing example usage, output, and commentary for this tool.
.SH OS
Linux
.SH STABILITY
Unstable - in development.
.SH AUTHOR
Brendan Gregg
.SH SEE ALSO
tcptop(8)
